Just make a mount point in /tmp or something and mount it.
On the console you should see a message when you plug the stick in. It usually comes up as da0.
Drop to a prompt and make the mount point- mkdir /tmp/usb
If it's a FAT stick from a Windows box, you need to mount as dos fs- mount_msdosfs /dev/da0s1 /tmp/usb
You can then cd to /tmp/usb and copy files or whatever. If you are running nano, remember to mount rw if you need to copy somewhere other then tmp or var.